Yes, sorry, I should have made that clear as regards Malwarebytes/AdWCleaner. Run those in normal boot mode of course.
My feeling from the fairly sparse and unclear information provided by the OP is that this is more likely some type of PUP running at startup rather than anything else.
In Safe Mode AVAST won't give any better detection but it might stop whatever is causing this problem running which, if it was, could prevent it from being removed. That's also why I suggested taking the PC offline too. It might be connecting to some dodgy site every time the machine boots.
I'd do a through clean up using CCleaner too and get rid of all the cookies for the same reason.